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1. Molecular composition of short-period comets from millimetre-wave spectroscopy: 21P/Giacobini-Zinner, 38P/Stephan-Oterma, 41P/Tuttle-Giacobini-Kresák, and 64P/Swift-Gehrels

   https://doi.org/10.1051/0004-6361/202140765  

   Biver, N.; Bockelée-Morvan, D.; Lis, D. C.; Despois, D.; Moreno, R.; Crovisier, J.; Colom, P.; Boissier, J.; Dello Russo, N.; Vervack, R. J.; et al (July 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present the results of millimetre-wave spectroscopic observations and spectral surveys of the following short-period comets: 21P/Giacobini-Zinner in September 2018, 41P/Tuttle-Giacobini-Kresák in April 2017, and 64P/Swift-Gehrels and 38P/Stephan-Oterma in December 2018, carried out with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m radio telescope at wavelengths between 1 and 3 mm. Comet 21P was also observed in November 1998 with the IRAM 30-m, James Clerk Maxwell Telescope, and the Caltech submillimeter Observatory radio telescopes at wavelengths from 0.8 to 3 mm. The abundances of the following molecules have been determined in those comets: HCN, CH 3 OH, CS, H 2 CO, CH 3 CN, and H 2 S in comet 21P; HCN and CH 3 OH in 41P; HCN, CH 3 OH, and CS in 64P; and CH 3 OH in 38P. The last three comets, classified as carbon-chain typical from visible spectro-photometry, are relatively rich in methanol (3.5–5% relative to water). On the other hand, comet 21P, classified as carbon-chain depleted, shows abundances relative to water which are low for methanol (1.7%), very low for H 2 S (0.1%), and also relatively low for H 2 CO (0.16%) and CO (<2.5%). Observations of comet 21P do not show any change in activity and composition between the 1998 and 2018 perihelions. Sensitive upper limits on the abundances of other molecules such as CO, HNCO, HNC, or SO are also reported for these comets. 

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2. Coma composition of comet 67P/Churyumov-Gerasimenko from radio-wave spectroscopy

   https://doi.org/10.1051/0004-6361/202245672  

   Biver, N.; Bockelée-Morvan, D.; Crovisier, J.; Sandqvist, Aa.; Boissier, J.; Lis, D. C.; Cordiner, M.; Bonev, B. P.; Dello Russo, N.; Moreno, R.; et al (April 2023, Astronomy & Astrophysics)

   We present the results of a molecular survey of comet 67P/Churyumov-Gerasimenko undertaken with the Institut de RadioAstronomie Millimétrique (IRAM) 30-m radio telescope in November–December 2021, when it had its most favourable apparition in decades. Observations at IRAM 30-m during the 12–16 November period covered 8 GHz bandwidth at 3 mm, 16 GHz at 2 mm, and 60 GHz in the 1 mm window domain. These were completed by snapshots at 1 mm on 12–13 December and a short observation of the H 2 O line at 557 GHz with the Odin sub-millimetre observatory on 17.0 November 2021, and with 18-cm observations of OH with the Nançay radio telescope. Less sensitive observations obtained at a previous perihelion passage on 18–22 September 2015 with IRAM and 9–12 November 2015 with Odin are also presented. The gas outflow velocity, outgassing pattern, and temperature have been accurately constrained by the observations. They are perfectly consistent with those measured in situ with the Rosetta/MIRO sub-millimetre instrument in 2015. In particular, the asymmetry of the line is well represented by a jet concentrating three-quarters of the outgassing in about π steradians. We derived abundances relative to water for seven molecules and significant upper limits for approximately five others. The retrieved abundances were compared to those measured in situ at the previous perihelion with Rosetta. While those of HCN, CH 3 OH, and HNCO are comparable, 67P is found to be depleted in H 2 S and relatively normal in CS (H 2 S/CS ≈ 3) in strong contradiction with the Rosetta/ROSINA mass spectrometer measurement of the H 2 S/CS 2 (≈100) abundance ratio. While the formaldehyde total abundance found with IRAM 30-m when assuming it to be mostly produced by a distributed source (Haser parent scale length ≈8000 km) is similar to the one derived by Rosetta/ROSINA, we find that the formaldehyde coming from the nucleus is one order of magnitude less abundant than measured in situ by Rosetta/ROSINA. 

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3. Coma Abundances of Volatiles at Small Heliocentric Distances: Compositional Measurements of Long-period Comet C/2020 S3 (Erasmus)

   https://doi.org/10.3847/1538-3881/ad0e02  

   Ejeta, Chemeda; Gibb, Erika; Roth, Nathan; DiSanti, Michael A; Dello_Russo, Neil; Saki, Mohammad; McKay, Adam J; Kawakita, Hideyo; Khan, Younas; Bonev, Boncho P; et al (December 2023, The Astronomical Journal)

   Abstract We report production rates of H₂O and nine trace molecules (C₂H₆, CH₄, H₂CO, CH₃OH, HCN, NH₃, C₂H₂, OCS, and CO) in long-period comet C/2020 S3 (Erasmus) using the high-resolution, cross-dispersed infrared spectrograph (iSHELL) at the NASA Infrared Telescope Facility, on two pre-perihelion dates at heliocentric distancesR_h= 0.49 and 0.52 au. Our molecular abundances with respect to simultaneously or contemporaneously measured H₂O indicate that S3 is depleted in CH₃OH compared to its mean abundance relative to H₂O among the overall comet population (Oort Cloud comets and Jupiter-family comets combined), whereas the eight other measured species have near-average abundances relative to H₂O. In addition, compared to comets observed atR_h< 0.80 au at near-infrared wavelengths, S3 showed enhancement in the abundances of volatile species H₂CO, NH₃, and C₂H₂, indicating possible additional (distributed) sources in the coma for these volatile species. The spatial profiles of volatile species in S3 in different instrumental settings are dramatically different, which might suggest temporal variability in comet outgassing behavior between the nonsimultaneous measurements. The spatial distributions of simultaneously measured volatile species C₂H₆and CH₄are nearly symmetric and closely track each other, while those of CO and HCN co-measured with H₂O (using different instrument settings) are similar to each other and are asymmetric in the antisunward direction. 

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4. The Interstellar Heritage of Comets

   https://doi.org/10.3847/1538-3881/ae0cae  

   Willacy, Karen; Majumdar, Liton; Bonev, Boncho; Gibb, Erika; Dello_Russo, Neil; DiSanti, Michael; Vervack, Ronald J; Roth, Nathan X (November 2025, The Astronomical Journal)

   Abstract Comets have similar compositions to interstellar medium ices, suggesting at least some of their molecules may be inherited from an earlier stage of evolution. To investigate the degree to which this might have occurred, we compare the composition of individual comets to that of the well-studied protostellar region IRAS 16293–2422B. We show that the observed molecular abundance ratios in several comets correlate well with those observed in the protostellar source. However, this does not necessarily mean that the cometary abundances are identical to protostellar. We find the abundance ratios of many molecules present in comets are enhanced compared to their protostellar counterparts. For COH molecules, the data suggest higher abundances relative to methanol of more complex species, e.g., HCOOH, CH₃CHO, and HCOOCH₃, are found in comets. For N-bearing molecules, the ratios of nitriles relative to CH₃CN—HC₃N/CH₃CN and HCN/CH₃CN—tend to be enhanced. The abundances of cometary SO and SO₂relative to H₂S are enhanced, whereas OCS/H₂S is reduced. Using a subset of comets with a common set of observed molecules, we suggest a possible means of determining the relative degree to which they retain interstellar ices. This analysis suggests that over 84% of COH-bearing molecules can be explained by the protostellar composition. The possible fraction inherited from the protostellar region is lower for nitrogen molecules, at only 26%–74%. While this is still speculative, especially since few comets have large numbers of observed molecules, it provides a possible route for determining the relative degree to which comets contain disk-processed material. 

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5. Molecular composition of comet 46P/Wirtanen from millimetre-wave spectroscopy

   https://doi.org/10.1051/0004-6361/202040125  

   Biver, N.; Bockelée-Morvan, D.; Boissier, J.; Moreno, R.; Crovisier, J.; Lis, D. C.; Colom, P.; Cordiner, M. A.; Milam, S. N.; Roth, N. X.; et al (April 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present the results of a molecular survey of comet 46P/Wirtanen undertaken with the IRAM 30-m and NOEMA radio telescopes in December 2018. Observations at IRAM 30-m during the 12–18 December period comprise a 2 mm spectral survey covering 25 GHz and a 1 mm survey covering 62 GHz. The gas outflow velocity and kinetic temperature have been accurately constrained by the observations. We derive abundances of 11 molecules, some being identified remotely for the first time in a Jupiter-family comet, including complex organic molecules such as formamide, ethylene glycol, acetaldehyde, or ethanol. Sensitive upper limits on the abundances of 24 other molecules are obtained. The comet is found to be relatively rich in methanol (3.4% relative to water), but relatively depleted in CO, CS, HNC, HNCO, and HCOOH. 

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